Fix #515: socket: excessively sparse array

This patch adds __serialize hook to sockets to fix REPL && admin console.
snap_daemon.test waits for snap_daemon done (or timeout).
Merge branch 'master' of github.com:tarantool/tarantool
box.net.box works with big packages properly. closes #518
Security update for libyaml

libyaml updated to upstream 0.1.6 (hg bce8b60f0b9a) in order to
fix heap overflow in yaml_parser_scan_uri_escapes.

This patch also revert our hack with extra identation of
flow-style sequences and maps at root level:

    setmetatable({10, 15, 20}, { __serialize="seq"})
    now encoded as
    "--- [10, 15, 20]\n...\n"
    instead of
    "---\n[10, 15, 20]\n...\n".

This trick is no more needed because REPL never uses flow mode
at root level.
Merge branch 'master' of https://github.com/tarantool/tarantool